In many areas of technology, there is a need to apply fluids—especially those comprising particle material—in thin layers on to a carrier. In many such cases, it is also necessary for the applied layers to have the smoothest possible surface.
For example, a smooth application of particle material to be bonded plays an important role in rapid prototyping procedures.
A rapid prototyping procedure for creating casting patterns is known from DE 198 53 834.0, for instance. In this case, untreated particle material like quartz sand is applied in a thin layer to an assembly platform. A spraying device is then used to finely distribute bonding agent all over the particle material. After that, a hardener is applied over selected sections in order to strengthen corresponding areas of the particle material. This process is repeated several times to individually mould a body from the bonded particle material. This body is initially embedded in the surrounding, unbound particle material, from where it can be subsequently removed following completion of the assembly phase.
If this type of rapid prototyping procedure involves the use of quartz sand as the particle material and furan resin as the bonding agent, for example, a hardener comprising sulphuric acid can be used to produce a mould comprising a common substance familiar to experts.
Difficulties arising during these well-known procedures are often posed by the smoothest and thinnest possible application of particle material; this influences the layer thickness, i.e. the smallest unit and therefore also the accuracy at which the mould can be produced.
One procedure of applying a layer of powder on a surface is known from EP 0 538 244 B1; in this case, the surface is fed with powder and then travelled over by a roller which rotates slowly in a direction opposing its linear travel over the surface. The powder makes contact with the roller rotating in the opposite direction and is thus distributed in a layer on the surface as the roller travels over it. This coating technique prevents any significant shear stress from being exerted on previously applied layers and the mould from being damaged as a result.
U.S. Pat. No. 5,902,537 also describes the application of particle material by means of a roller rotating in a direction opposing its linear propagation.
However, if such coating techniques involve the use of powders with a strong tendency to form agglomerates, for instance, particle material possessing a very fine grain or a content of binder, it proves very difficult to apply the material in a thin and smooth layer. The particle material tends to form lumps which stick to the roller, thus preventing achievement of a smooth surface.
Given the use of particle material—especially with a tendency to form lumps—a counter-rotating roller also has the disadvantage of heavily soiling all components which make contact with the particle material, thus entailing more frequent maintenance and proving more expensive.
Similarly, the coating technique described in U.S. Pat. No. 5,730,925 is not capable of achieving smooth surfaces with powders tending to agglomerate, because the powders also form lumps and result in rough surfaces in this case.
U.S. Pat. No. 6,036,777 describes a device for applying powder to a surface. A distributor moving across the surface to be coated applies layers of powder on it. The distributor operates in conjunction with a vibrating mechanism for compacting the powder.
This vibrating mechanism also proves disadvantageous, the vertical exertion of force on the particle layer producing an undesirable compression effect which causes uneven compaction of the required mould or model. This results in uncontrolled displacements in the mould during its fabrication on the powder bed, thus impairing the production accuracy.
Under certain circumstances, particle material with a strong tendency toward agglomeration might even make it completely impossible to produce smooth layers.
However, it is precisely a generation of smooth powder layers which proves extremely important in many applications. In the case of the rapid prototyping procedure described in some detail earlier, it is essential to apply particle layers as thinly and evenly as possible in order to maximize the accuracy of component production. Layer thickness constitutes the smallest possible increment during step-by-step component build-up. The thicker and rougher the layers, the less accurate the shape of the final cast.